King Street Bridge failure

Fatigue crack at weld toe leading to brittle fracture through web; top flange intact due to deflection-limited geometry. Tom uses it as the opening case for the day; references it as discussed the prior session.

This is the King Street Bridge I mentioned yesterday. Here is the weld detail on one of the girders that was holding up the bridge. They developed a fatigue crack at the toe of this weld — lots of restraint as you go around — developed a fatigue crack, and then you had a brittle fracture go up through there. Like I said, no one got hurt or killed. In fact, as you look at the picture, the thing didn't even break all the way through; the top flange didn't break through.

Melbourne, Australia, 1960s. Plate-on-plate fillet weld run all the way around the closure produced a fatigue crack that brought the bridge down. Now a forbidden detail in the codes — illustrates the principle that codes encode failure history. Tom does not recall whether anyone was killed.

So there's lots of little details and games that we have learned to do, because we have learned that some little details are terrible. There is a story of the King Street Bridge in Melbourne, Australia back in the 60s. They had a plate on top of another plate. So this plate was on another plate, and they made the weld all the way around. This might be four inches, this might be twelve inches over here. They just made a fillet weld all the way around. Fatigue crack came along, the bridge fell down, a bunch of cars went in the river. I don't think anybody got killed, I don't remember, but in any case it's a fairly famous failure.

Classic case of a doubler plate welded all around its tip, creating a fatigue crack initiation site at the change in stress concentration. Used to motivate the AISC fatigue-class restrictions on wrap-around welds.

In fact this type of detail right here, where they weld the end of the doubler plate they put on to stiffen up the beam — that's the reason for the King Street Bridge failure in Australia, which is sort of a famous weld failure. I don't think anybody got killed, but it was sort of a major bridge in Sydney or somewhere. It just cracked. They had a doubler plate and they had welded all around the tip. And people learned, oh, that's not a good design detail, because you'll start a fatigue crack right there at the change in stress concentration.

Cruciform fillet weld detail with full wrap-around at a doubler plate; hydrogen crack at the high-restraint location grew in fatigue on the tension side until the bridge collapsed. Used as the case for why modern codes prohibit wrapping welds around the ends.

I had mentioned the King Street Bridge failure in Melbourne, Australia, and I was looking through a book on design of weldments for this course and came across the design that caused the failure. Here's the girder which is holding up the bridge — a very big girder; they didn't give me the dimensions. You had a plate and another plate, and they put fillet welds along here. On one side you have a fillet weld, and on the other side they had what they call a doubler plate. You need a little more stiffness on the flange, so they put a doubler plate on. I had mentioned to you they had welded all the way around.

Brittle fracture of welded bridge in Melbourne, late 1970s. Doubler-plate detail with weld going across the loaded direction; hydrogen crack from welding grew by fatigue to critical size. Major thoroughfare lost for ~1 year. Used to justify the welding-detail prohibition (don't weld around the corners).

You have to pay attention to design for fatigue. This is the King Street Bridge in Melbourne, Australia. I think it failed in the late 1970s. Cars going across the bridge, a major thoroughfare. Brittle fracture going out through here. Here's your weld detail — doubler plate, on the bottom flange, probably a cover plate. It had a hydrogen crack from welding, grew by fatigue to a critical size, and then bang. They lost a major thoroughfare for about a year. Since then you're not allowed to put the weld going across that direction — you weld in this direction and this direction, because the corners are tremendous sources of hydrogen cracks and fatigue. Certain types of details are forbidden, not because someone calculated it one day, but because something failed. That's how codes and standards are developed — it's experience. Someone has a failure, we go study why.

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